Ryan and Republican presidential candidate Mitt Romney have proposed to lower all individual income taxes by 20 percent, while keeping the plan revenue neutral by ending deductions for high-income individuals. As many have pointed out, the plan is mathematically impossible — in order to keep the tax breaks revenue neutral, popular deductions that primarily affect the middle-class would also have to go.

Ryan, who is known as a numbers-crunching budget wonk, would presumably be able to explain how the plan would work. But pressed by host Chris Wallace to explain how the tax plan would add up, Ryan ducked.

RYAN: We’re saying, limited deductions so you can lower tax rates for everybody. Start with people at the higher end…lowering tax rates by broadening the tax base works.

WALLACE: You haven’t given me the math.

RYAN: (laughing) Well, I don’t have the time. It would take me too long to go through all the math. But let me say it this way, you can lower tax rates by 20 percent across the board by closing loopholes and still have preferences for the middle class for things like charitable deductions, for home purchases, for health care…

WALLACE: If — just suppose — that the doubters are right, President Romney takes office and the math doesn’t add up…

RYAN: First of all, run the numbers. They’ve run them in Congress. We’ve got five other studies that show you can do this.